Accroding to http://en.cppreference.com/w/cpp/memory/unique_ptr/reset ,
void reset( pointer ptr = pointer() );
template< class U >
void reset( U ) = delete;
void reset( std::nullptr_t p );
1) Given
current_ptr
, the pointer that was managed by*this
, performs the following actions, in this order: Saves a copy of the current pointerold_ptr = current_ptr
; Overwrites the current pointer with the argumentcurrent_ptr = ptr
; If the old pointer was non-empty, deletes the previously managed objectif(old_ptr != nullptr) get_deleter()(old_ptr)
.2) In the specialization for dynamic arrays,
std::unique_ptr<T[]>
, this template member is provided to prevent usingreset()
with a pointer to derived (which would result in undefined behavior with arrays).3) In the specialization for dynamic arrays,
std::unique_ptr<T[]>
, the third overload is necessary to allow reset tonullptr
(which would otherwise be prohibited by the template overload). Equivalent toreset(pointer())
Now that reset(nullptr)
is equivalent to reset(pointer())
, why does the latter exist?
If I want to reset an array form unique_ptr, why can I not just use rest(pointer())
?
The
template< class U >
void reset( U ) = delete;
would be chosen for a call with nullptr
argument, if not for
void reset( std::nullptr_t p );
That's why it exists, to allow the call with nullptr
.
Example (compile with FIX
defined to suppress the compilation error):
#include <cstddef> // std::nullptr_t
struct S
{
void reset( char* ) {}
template< class Type >
void reset( Type ) = delete;
#if FIX
void reset( std::nullptr_t ) {}
#endif
};
auto main() -> int
{
S().reset( nullptr ); // Fails when FIX is not defined.
}
reset
ist implemented as
pointer old = this->ptr;
this->ptr= newPointer;
delete[] old;
The templated overload is deleted for arrays to prevent the following case
class foo{};
class bar : public foo {};
foo* managedPointer = new foo[3];
bar* newPointer = new bar[5];
foo* old = managedPointer;
managedPointer = newPointer;
delete[] old;
Which is undefined behavior. Section 5.3.5 paragraph 3:
[...] In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.
Since deleted functions still participate in overload resolution and reset(U)
provide a better match for nullptr
than reset(pointer)
, there is an additional overload to to allow reset(nullptr)
, which would otherwise give a compiler error and thus result in an inconsistent interface between array and pointer version.
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